This invention relates to fluid-gauging systems.
The invention is particularly concerned with gauging systems for providing an indication of the quantity of fuel in a fuel tank and, more particularly, a fuel tank in an aircraft.
In previous fluid-gauging systems, the level of fluid in a container is determined by detecting changes in the value of a capacitance sensor located within the container. The sensor generally has two plate electrodes, separated by an air-gap which is filled or emptied as the level of fluid changes, thereby altering its capacitance. An alternating electric signal is supplied to the sensor, and its output is supplied to a suitable measuring or detecting circuit.
Difficulties have been experienced in the past with such systems for various reasons. In some previous arrangements it has been necessary to use electrically-screened cables to supply the energizing signal to the sensor capacitor and to supply the output signal from the capacitor to the measuring or detecting circuit. Without such screened cables there is a risk that there will be electrical interference with these signals from other electrical equipment in the vicinity. Screened cables, however, have the disadvantage of being substantially heavier and more expensive than unscreened cables; this advantage is especially important in aircraft applications and even more so where a large number of sensing capacitors are used each of which may be provided with an individual cable. Connectors used with screened cables must also themselves be screened and this leads to a further increase in complexity, cost and weight.
In an attempt to overcome the need to provide screened cables, it has been proposed to rectify the output from the sensing capacitor by means of two diodes connected in opposite senses to one electrode of the capacitor and mounted close to the capacitor within the fuel tank or other container. Each diode produces a half-wave rectified signal that is supplied via respective unscreened leads to the detecting circuit. The use of diodes in this way also avoids the need to use screened cables for supplying the energisation signal to the sensor capacitor. There are, however, disadvantages with such an arrangement in that the performance of the diodes and, in particular, the voltage drop across the diodes, varies with change of temperature. Compensation for these temperatures effects can be provided by, for example, deriving separate signals from other diodes mounted in the tank or container and using these separate signals for compensation. Alternatively, the temperature effects can be reduced by supplying a relatively high voltage across the sensing capacitor.
Another problem that arises with capacitive fuel-gauging systems is that the presence of electrically-conductive contaminants, such as water, can significantly affect the output of the sensor and lead to severe errors in the fuel reading. The presence of, for example, one liter of water would normally cause the fuel reading to be erroneously increased by several times this value.